1. Field of the Invention
The present invention relates to a method of measuring a natural frequency of a motor. The measuring method is applicable to, for example, a case where check is made as to if a prepressure that is applied in advance to the ball bearings therein is a preset value.
2. Related Art
In some of the conventional motors, the ball bearings are used for rotatably supporting a rotor by a stator. In this type of the motor, a load (a proper load determined substantially by the ball bearings) is applied to the ball bearings in a given direction in order to secure an accurate positioning in the radial and axial directions, to suppress a swing, to increase a rigidity of the ball bearings, and to improve an accuracy of the rotation by stabilizing the path of the ball bearings.
Such a mounting way is called a prepressure. The prepressure is categorized into a fixed position prepressure in which a relative position of the ball bearing in the axial direction is invariable even when it is used, and a fixed pressure prepressure in which the quantity of the prepressure can be kept substantially constant by making use of an elastic member, such as a coiled spring, even if a relative position of the ball bearing in the axial direction is varied, even when it is used.
In the fixed position pressure, the inner ring and the outer ring of the ball bearing are both fixed by adhesive. In the fixed pressure prepressure, either of the inner ring or the outer ring of the ball bearing is not fixed, or slidable.
A insufficient quantity of the prepressure applied to the ball bearing causes a play in the rotor. An excessive quantity of the prepressure causes an overheat, increase of a friction moment, reduction of fatigue life, and other problems.
Therefore, the quantity of the prepressure applied to the ball bearing must be selected to be proper in consideration of use conditions, the purpose of the prepressure, and the like.
For this reason, it is necessary to check as to if a preset prepressure is applied to the ball bearing. Some methods of the check are known.
1) In a first check method, a motor is vibrated and a natural frequency of the motor is measured. In this method, when a measured natural frequency is higher than a preset one, it is considered that the quantity of the prepressure is larger than desired. When the former is lower than the latter, it is considered that the quantity of the prepressure is smaller than desired.
2) In a second check method, a current value for driving a motor is measured. When a measured current value is larger than a preset one, it is considered that the prepressure is higher than desired. When the measured current value is smaller, it is considered that the prepressure is smaller than desired.
3) In a third check method, after the motor is driven, the feed of current is ceased, and a time period where the motor stops is measured. When a measured motor-stop period of time is shorter than a preset one, it is considered that the quantity of the prepressure is larger than desired. When the motor-stop time period is longer, it is considered that the quantity of the prepressure is smaller than desired.
In the first check method in which a natural frequency of the motor is measured, and the prepressure to the ball bearing is checked, to vibrate the motor, the motor is hit with a hammer or set to a vibrator machine.
When the motor is hit with a hammer, the motor may be damaged and the ball bearing contained therein may also be damaged.
Where the vibrator machine is used for vibrating the motor, the machine must be installed, leading to increase of cost.